Ever since I saw the photo on the house in the background in the pic at the picnic I've thought that the MCs mom will be related to Jill in some way, the only thing that has been throwing me is the 'age gap', although I'm not sure how old Jill is, but in the flashbacks Jill looks between 6-8 years old and we know the MC just had his 19th birthday, I guess the question is, is Jill the aunt or the cousin, cousins in most countries is OK, except the US which is morally about 1000years behind the times. I guess the MC will find out by reading more from his mom's diary.